Vacuum forming by removing the air from between thin plastic sheet and a mold is well known. Such a process is utilized in forming household refrigerator liners which are relatively large. In the usual vacuum forming process the mold is made of aluminum and the plastic sheet is heated and blown into a bubble. The mold is then placed inside the bubble and a vacuum is drawn so that the bubble collapses to conform to the shape of the mold. Upon cooling the plastic material solidifies and permanently assumes the shape of the mold. The air is pulled or vacuumed through small holes over most of the surface of the mold and sucked into a large vacuum storage tank by a vacuum pump. In such a process the plastic material forming areas of the back of the liner contact the mold and solidify first. As the remaining air is removed from the areas forming the top, sides, and bottom of the liner, the plastic material is stretched and thinned and as the thinned plastic material is drawn down against the mold the remaining plastic material has to stretch further. In order to have the complete article properly formed it is important that the sheet of plastic material has been heated to the proper elevated temperature for the molding process so that the material will flow and "relax" and thus relieve the stresses in the plastic material that could cause subsequent cracking. This is particularly so in the last portion of the drawn article and in the case of liners for household refrigerators it is the flanged periphery at the open front of the liner that has the area of high forming stresses which is subject to stress cracking. It is very difficult to accurately measure the temperature of the sheet of plastic material to assure that it has reached the proper temperature for molding and therefore it is desirable to be able to provide a means for visually inspecting the final formed plastic article to see if in fact the temperature of the sheet of plastic material was correct and therefore provided adequate flow of the material during the molding process. This is important because subsequent use of the article such as a refrigerator liner if not properly formed in the high stress areas such as the peripheral flange can result in cracking some time during or subsequent to assembly of the liner into the refrigerator. It is therefore desirable to determine if the liner has been formed correctly prior to assembly of the liner into a refrigerator. Prevention of using improperly formed liners in refrigerators saves material cost and expensive rework in the assembly line of the refrigerator. This also prevents improperly formed liners from being built into refrigerators. These liners could appear satisfactory but subsequently crack when subjected to very low temperatures, such as during shipping in very cold weather. By this invention a process is provided that allows visual inspection of a deep drawn plastic article having high forming stress areas to determine if the plastic material was formed at the correct elevated temperature.